This invention relates to the field of two-way communication between a base station and a number of remote terminal units and, more particularly, to a polling system with increased channel efficiency.
Many types of communications systems are known in which a base station interrogates individual remote stations, the most common method being transmission of one unique address code, followed by a pause in which the base station listens for the addressed unit to respond, then transmitting a second unique address code, pausing, etc. In U.S. Pat. No. 4,251,865 issued Feb. 17, 1981, having the same assignee as the present invention, a polling arrangement was disclosed having two features. First, only those terminal units tagged as "active" are regularly polled. In this context, active is defined as presently transmitting data or processing data, or having done either within a brief preceding period of time; e.g., the previous five minutes. "Inactive"units are also polled, but at much less frequent intervals, and only a few during each polling cycle. Even in this system, many address codes are transmitted which are not responded to.
As to the second feature of the copending application, each remote terminal having data to transmit would immediately respond with the address code of the base station. When the base station received and detected its address code, it immediately aborted the address code it was transmitting at that time and waited until the responding unit's message was complete before resuming the polling sequence with the aborted code. The repsonding unit was identified by the address code immediately preceding the aborted code. Neither base nor terminals used duplex operation.